In a year when you can’t turn around without tripping over half a dozen computer-animated comedies, each more derivative than the last, it’s a little sad at first to find that Aardman Animations have fallen onto the bandwagon. Aardman probably aren’t that well known outside the UK, but over here they’re something of an institution, almost entirely on the back of the Wallace and Gromit series of films. Up until now they’ve worked mostly in “claymation”, which is pretty much what it sounds like – stop-motion using modelling clay – and Flushed Away is their first CGI feature, as well as their first film to extensively use Hollywood voice talent.

It’s definitely a departure, but not necessarily the sell-out that it looks like at first glance. For a start, Flushed Away retains a lot of the look of their previous work, in terms of character design and also in its ridiculous attention to detail. It also doesn’t rely at all on the celebrity name-dropping that has assailed similar films, with most of the cast barely recognisable – Hugh Jackman sounds more like Hugh Grant, Kate Winslet manages a cockney accent, and in general it seems like everyone’s been cast for reasons other than how big their names can be printed on the poster. The animation and voice acting are very good, but they don’t try to carry the film.

Neither, for that matter, does the story. Flushed Away revolves around Jackman’s hero Roddy, a pet rat who finds himself rudely ejected from his cosy life as a pet in a posh townhouse, (via the toilet – hence the title), and cast down into the subterranean underworld of London’s sewers. He soon discovers that his only way home lies with Rita (Winslet) and her boat The Jammy Dodger – but Rita has problems of her own, with amphibian arch-criminal The Toad, his rodent henchmen, and his assassin cousin Le Frog.

It’s all about as daft as it sounds, but in a good way. The humour is broader and probably less Anglo-centric than in the Wallace and Gromit films, but often funnier just because it tries so damn hard. Flushed Away takes the time-honoured approach of hurling jokes at the screen and hoping most of them hit – some are lousy, some are inspired, but there’s absolutely no letup and little effort to cram in plot, characterisation or Disney-style moral education. There isn’t even much of a romantic sub-plot – which is probably a good thing given how disturbing that would be with rats, even vaguely anthropomorphised ones. Instead, you get a movie that barely lets you pause for breath, which veers randomly from sight gags to surrealism to toilet humour, and gets away with it by having more jokes that work than don’t.

Ultimately, Flushed Away is a little too slight to be a classic comedy, lacking anything like Wallace and Gromit’s sly social commentary to give it lasting depth. But it has a few moments of undeniable genius, (a Greek chorus of singing slugs, anyone?), and as a fun diversion it works just fine – you’ll be hard pressed to find something in its avalanche of jokes that doesn’t make you laugh.